Production process of Mn-Zn ferrite

ABSTRACT

The present invention provides a production process of a Mn—Zn ferrite that enables wastes of sintered cores to be recycled without serious difficulties in sintering. The production process comprises recycling a powder obtained by milling a sintered Mn—Zn ferrite, thereby obtaining a sintered core having a component composition including 44.0 to 49.8 mol % Fe 2 O 3 , 4.0 to 26.5 mol % ZnO, 0.02 to 1.00 mol % Mn 2 O 3  and a remainder MnO.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a production process of a Mn—Znferrite, and more particularly to a production process of a Mn—Znferrite that enables wastes of sintered products to be recycled.

[0003] 2. Description of the Related Art

[0004] Typical oxide magnetic materials having soft magnetism include aMn—Zn ferrite that has been used as a low loss material used forswitching power transformers, flyback transformers and the like, variousinductance elements, an impedanace element for EMI countermeasures, anelectromagnetic wave absorber and the like. Conventionally, this Mn—Znferrite usually has a basic component composition containing over 50 mol% (52 to 55 mol % on the average) Fe₂O₃, 10 to 24 mol % ZnO and theremainder MnO. The Mn—Zn ferrite is usually produced by mixingrespective material powders of Fe₂O₃, ZnO and MnO in a prescribed ratio,subjecting mixed powders to the respective steps of calcination,milling, component adjustment, granulation and pressing to obtain adesired shape, then performing sintering treatment at 1200 to 1400° C.for 3 to 4 hours in a reducing atmosphere in which a relative partialpressure of oxygen is considerably lowered by supplying nitrogen. Thereason why the Mn—Zn ferrite is sintered in the reducing atmosphere isthat when the Mn—Zn ferrite containing over 50 mol % Fe₂O₃ is sinteredin the air, densification is not attained sufficiently thereby failingto obtain excellent soft magnetism, and that Fe²⁺ which has positivecrystal magnetic anisotropy is formed by reducing a part of Fe₂O₃exceeding 50 mol % thereby canceling negative crystal magneticanisotropy of Fe³⁺ and enhancing soft magnetism.

[0005] Amount of the above-mentioned Fe²⁺ formed depends on relativepartial pressures of oxygen in sintering and cooling after thesintering. Therefore, when the relative partial pressure of oxygen isimproperly set, it becomes difficult to ensure excellent soft magneticproperties. Thus, conventionally, the following expression (1) has beenexperimentally established and the relative partial pressure of oxygenin sintering and in cooling after the sintering has been controlledstrictly in accordance with this expression (1).

log Po₂=−14540/(T+273)+b  (1)

[0006] where T is temperature (° C.), Po₂ is a relative partial pressureof oxygen, and b is a constant, which is usually 7 to 8.

[0007] In addition, the above-mentioned milling step is conducted sothat an average grain size of a fine milled powder ranges 1.0 to 1.4 μm.If the average grain size is more than 1.4 μm, a desired density can notbe obtained in sintering, and if the average grain size is less than 1.0μm, it becomes difficult to handle the resultant powder.

[0008] A large amount of wastes are generated for several reasons, suchas a surplus, defects and the like in each step of the productionprocess of Mn—Zn ferrite described above. While wastes generated priorto the step of pressing can be recycled without particular problems,wastes generated in the step of sintering due to defects, such asdimensional error, cracking, chipping or the like, are difficult torecycle for the reason described below and are just scrapped as theyare.

[0009] The step of sintering a Mu—Zn ferrite is largely affected byvacancy concentration of oxygen ions that have the lowest diffusing rateamong its constituent ions. As the vacancy concentration of oxygen ionsincreases, the diffusion of oxygen ions, iron ions, manganese ions andzinc ions is accelerated and the sintered density increases. Fe₂O₃content and a relative partial pressure of oxygen in an atmosphere arefactors governing the vacancy concentration of oxygen ions. The smallerthe Fe₂O₃ content is and the lower the relative partial pressure ofoxygen is, the easier the vacancies of oxygen ions can be formed.Because a conventional Mn—Zn ferrite contains over 50 mol % Fe₂O₃, thevacancies of oxygen ions decrease, whereas the respective vacancies ofiron ions, manganese ions and zinc ions increase. That is, in case aconventional sintered Mn—Zn ferrite is milled and pressed for recycling,it must be sintered with the relative partial pressure of oxygen in anatmosphere considerably lowered. However, the lowest relative partialpressure of oxygen available in actual mass production process is about0.0001 in which a desired vacancy concentration of oxygen can not beobtained. As a result of this, the sintering can not be conductedsmoothly making it difficult to obtain a desired density. Consequently,the resultant sintered cores do not have magnetic properties good enoughto serve for practical use and therefore are simply scrapped.

SUMMARY OF THE INVENTION

[0010] The present invention has been made in consideration of theabove-mentioned conventional problems, and an object of the presentinvention is therefore to provide a production process of a Mn—Znferrite, which enables wastes of sintered cores to be recycled withoutserious difficulties in sintering.

[0011] In order to attain the above-mentioned object, a productionprocess of a Mn—Zn ferrite according to the present invention comprisesthe steps of: milling a sintered core of Mn—Zn ferrite for recycling;subjecting a recycled powder to a component adjustment so as to have acomposition of 44.0 to 49.8 mol % Fe₂O₃, 4.0 to 26.5 mol % ZnO, 0.02 to1.00 mol % Mn₂O₃ and a remainder being MnO; pressing a mixed powdersubjected to the component adjustment; and sintering a green compactobtained by pressing the mixed powder.

[0012] Amount of powder to be recycled, that is a recycled powder, canbe arbitrarily selected. When the recycled powder has a target componentcomposition, all mixed powder for pressing may be recycled And, when therecycled powder does not have a target component composition, thecomponents must be adjusted by appropriately adding respective rawmaterial powders of Fe₂O₃, ZnO, MnO or the like.

[0013] As Fe₂O₃ content is restricted to less than 50 mol % in thepresent invention as mentioned above, vacancies of oxygen ions areeasily formed in the sintering step and the density of a sintered coreis easily increased. Therefore, when the sintering (heating—maintainingtemperature—cooling) is conducted in an atmosphere containing anappropriate amount of oxygen, the resultant sintered core hassufficiently high density even if a recycled powder is used. However, astoo small Fe₂O₃ content results in lowering the initial permeability, atleast 44.0 mol % Fe₂O₃ must be contained in the ferrite.

[0014] ZnO influences the Curie temperature and saturationmagnetization. Too large amount of ZnO lowers the Curie temperature toresult in practical problems, but on the contrary, too small amount ofZnO reduces the saturation magnetization, so it is desirable for ZnOcontent to be set to the above-mentioned range of 4.0 to 26.5 mol %.

[0015] A manganese component in the above-mentioned ferrite exists asMn²⁺ and Mn³⁺. Since Mn³⁺ distorts a crystal lattice: therebysignificantly lowering the initial permeability, Mn₂O₃ content is set to1.00 mol % or less. However, since too small Mn₂O₃ content lowers theelectrical resistivity significantly, at least 0.02 mol % Mn₂O₃ must becontained in the ferrite.

[0016] It is desirable for the lower limit of the average gram size ofthe recycled powders to be set to about 1.0 μm similarly to the priorart, However, even when the average grain size exceeds 1.4 μm andmeasures, for example about 2.0 μm, sufficiently high density isobtained in the sintering.

[0017] Since the present invention relates to recycling of wastes ofWintered cores, a recycled ferrite naturally contains additivescontained in the wastes of sintered cores. Generally, CaO, SiO₂, ZrO₂,Ta₂O₅, HfO₂, Nb2O₅, V₂O₅, Bi₂O₃, In₂O₃, CuO, MoO₃, WO₃, Al₂O₃, TiO₂ andSnO₂ are used as additive. Therefore, the recycled ferrite in thepresent invention may contain a slight amount of one or more of theseadditives.

[0018] In the present invention, the above-mentioned Mn₂O₃ content canbe controlled by sintering in an atmosphere of an adjusted relativepartial pressure of oxygen. In this case, it is desirable to control theMn₂O₃ content, that is the amount of Mn³⁺, by sintering and coolingafter the sintering in an atmosphere of a relative partial pressure ofoxygen obtained by using an arbitrary value in a range of 6 to 10 as theconstant b in the expression (1). When a value larger than 10 isselected as the constant b, the amount of Mn³⁺ in the ferrite exceeds 1mol %, whereby the initial permeability is rapidly lowered. Therefore,the amount of Mn³⁺ in the ferrite must be reduced to increase theinitial permeability. Thus, it is desired that a small value be selectedas the constant b. However, when a value smaller than 6 is selected,Fe²⁺ increases or Mn³⁺ decreases too much, thereby significantlylowering the electrical resistivity. Accordingly, the constant b is setto 6 at smallest. A relative partial pressure of oxygen (Po₂) may be setto a range of 0.0001 to 0.21, where the upper limit of 0.21 correspondsto the atmospheric pressure, and the lower limit of 0.0001 can beobtained in actual production process without serious difficulty.

[0019] In the present invention, Fe₂O₃ content is restricted to lessthan 50 mol % and a constant b in the expression (1) is set to anarbitrary value selected from a range of 6 to 10 as mentioned above.Therefore, the electrical resistivity of the resultant Mn—Zn ferrite is10 Ωm or more that is much higher than that of the conventional Mn—Zn(about 0.01 to 1 Ωm). Thus, for example, the Mn—Zn ferrite of thepresent invention is suitable for a magnetic material used in a highfrequency region exceeding 1 MHz.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] In production of the Mn—Zn ferrite, wastes of sintered Mn—Znferrite generated in sintering step are milled with an appropriatemilling measures, for example a hammer mill and a jet mill to obtain arecycled powder, and the respective raw material powders of Fe₂O₃, ZnO,MnO and the like as main components are mixed with the recycled powderin a prescribed ratio to obtain a mixed powder having a target componentcomposition. The recycled powder does not have to be grained fine at thebeginning, and may have an average grain size of about 40 μm or less. Inthis case, the mixed powder described above is calcined, then finelymilled to an average grain size of about 2 μm or less. The temperaturefor the calcination can be appropriately selected from a range of 850 to950° C. depending on a target composition. However, if the amount of theraw material powders to be added to the recycled powder is slight, thecalcination can be omitted. Further, a general-purpose ball mill can beused for the fine milling of the calcined powder. Then, the respectivepowders of several additives described above are mixed as required withthe fine mixed powders in a prescribed ratio to obtain a mixture havinga target component composition. Subsequently, the mixture is granulatedand pressed in accordance with a usual ferrite production process, thensintered at 1200 to 1400° C. for 2 to 4 hours

[0021] In the above-mentioned sintering and cooling after the sintering,a relative partial pressure of oxygen is controlled by flowing inert gassuch as nitrogen gas or the like into a sintering furnace. In this case,the constant b in the expression (1) can be arbitrarily set to a valueselected from a range of 6 to 10. Further, in this case, since thereaction of oxidation or reduction can be neglected independent ofrelative partial pressures of oxygen at a temperature of below 500° C.,the cooling after the sintering is to be conducted in accordance withthe above-mentioned expression (1) only till the temperature gets downto 500° C.

EXAMPLES Example 1

[0022] Respective raw material powders of Fe₂O₃, MnO and ZnO wereweighed for a composition of 53.0 mol % Fe₂O₃, and the remainderincluding MnO and ZnO with a molar ratio of MnO to ZnO being 3:2, andmixed with a ball mill. Then, the mixed powder was calcined in the airat 900° C. for 2 hours and further milled with a ball mill to obtain afine milled powder having an average grain size of 1.2 μm. Then, 0.05mass % CaO was added to this fine milled powder as additive and theadjusted mixture was further mixed with a ball mill for 1 hour. Then,this mixture was granulated with addition of polyvinyl alcohol, andpressed at a pressure of 80 MPa into toroidal cores (green compacts)each having an outer diameter of 18 mm, an inner diameter of 10 mm and aheight of 4 mm. The green compacts were placed in a sintering furnacewhere an atmosphere was adjusted by flowing nitrogen so as to have sucha relative partial pressure of oxygen as obtained by setting theconstant b in the expression (1) to 8, sintered at 1300° C. for 3 hoursand cooled after the sintering, and a sintered core (comparison sample1-1) equal to a conventional Mn—Zn ferrite was obtained.

[0023] Then, the sintered core (comparison sample 1-1) was milled with ahammer mill and a jet mill so as to have an average grain size of 40 μmor less to obtain a recycled powder. Then, the recycled powder wasmilled with a ball mill to obtain a mixed powder having an average grainsize of 1.2 μm. Then, this mixed powder was granulated with addition ofpolyvinyl alcohol, and pressed at a pressure of 80 MPa into toroidalcores (green compacts) each having an outer diameter of 18 mm, an innerdiameter of 10 mm and a height of 4 mm. The green compacts were placedin a sintering furnace where an atmosphere was adjusted by flowingnitrogen so as to have such a relative partial pressure of oxygen asobtained by setting the constant b in the expression (1) to 8, sinteredat 1300° C. for 3 hours and cooled after the sintering, and a recycledsintered core (comparison sample 1-2) having the same componentcomposition as a conventional Mn—Zn ferrite was obtained.

[0024] On the other hand, the sintered core (comparison sample 1-1) wasmilled with a hammer mill and a jet mill so as to have an average grainsize of 40 μm or less to obtain a recycled powder in the same manner asthe above, and respective raw material powders of MnO and ZnO were addedto the recycled powder so as to obtain a composition of 49.0 mol %Fe₂O₃, and the remainder including MnO, Mn₂O₃ and ZnO with a molar ratioof MnO to ZnO being 3:2 (both MnO and Mn₂O₃ are counted as MnO) toobtain a mixed powder. Then, this mixed powder was mixed with a ballmill and calcined at 900° C. for 2 hours. Further, the calcined powderwas milled with a ball mill to obtain two different fine milled powdershaving an average grain size of 1.2 μm and 2.0 μm, respectively. Then,these fine milled powders were both granulated with addition ofpolyvinyl alcohol, and pressed at a pressure of 80 MPa into toroidalcores (green conapatts) each having an outer diameter of 18 mm, an innerdiameter of 10 mm and a height of 4 mm. The green compacts were placedin a sintering furnace where an atmosphere was adjusted by flowingnitrogen so as to have such a relative partial pressure of oxygen asobtained by setting the constant b in the expression (1) to 8, sitteredat 130° C. for 3 hours and cooled after the sintering, and respectivesamples 1-3 and 1-4 of the present invention were obtained.

[0025] Final component compositions of the samples 1-1 to 1-4 thusobtained were checked by a fluorescent X ray analysis, and aquantitative analysis of Mn₂O₃ for the samples 1-3 and 1-4 of thepresent invention was conducted by a titration method. In addition, thesintered density and the initial permeability at 1 MHz were measured.The results are shown together in Table 1. TABLE 1 Basic ComponentAverage Initial Composition Grain Sintered Perme- Electrical Sample (mol%) Size Density ability Mn₂O₈ Resistivity No. Classification Fe₈O₃ MnO *ZnO (μm) (kg/m³) 1 MHz (mol %) (Ωm) 1-1 Comparison 53.0 28.2 18.8 1.24.97 × 10³ 1280 — 0.1 1-2 Comparison 68.0 28.2 18.8 1.2 4.42 × 10² 600 —0.1 1-3 Present 49.0 30.6 20.4 1.2 4.90 × 10⁸ 1240 0.47 110 Invention1-4 Present 49.0 30.6 20.4 2.0 4.88 × 10² 1280 0.48 110 Invention

[0026] As apparent from the results shown in Table 1, the comparisonsample 1-2 having the same component composition as a conventional Mn—Znferrite has lower density than the comparison sample (brand-new sinteredcore) 1-1 made from virgin raw material powders and has its initialpermeability lowered significantly, which makes the sample 1-2 useless.On the other hand, the samples 1-3 and 1-4 of the present invention,which are recycled, have densities and initial permeabilities equivalentto those of the comparison sample 1-1 of the brand-new sintered core.Therefore, it is clear that the production process of the presentinvention contributes greatly to the recycling of wastes of sinteredcores.

Example 2

[0027] The comparison sample 1-1 in Example 1 was milled with a hammermill and a jet mill so as to have an average grain size of 40 μm or lessto obtain a recycled powder. Then, respective raw material powders ofMnO and ZnO were added to the recycled powder so as to obtain acomposition of 49.0 mol % Fe₂O₃, and the remainder including MnO, Mn₂O₃and ZnO with a molar ratio of MnO to ZnO being 3:2 (both MnO and Mn₂O₃are counted as MnO), to thereby obtain a mixed powder. Then, this mixedpowder was mixed with a ball mill and calcined at 900° C. for 2 hours.Further, the calcined powder was milled with a ball mill to obtain afine milled powder having an average grain size of 1.2 μm. Then, thisfine milled powder was granulated with addition of polyvinyl alcohol,and pressed at a pressure of 80 MPa into toroidal cores (green compacts)each having an outer diameter of 18 mm, an inner diameter of 10 mm and aheight of 4 mm. The green compacts were placed in a sintering furnacewhere an atmosphere was adjusted by flowing nitrogen so as to have sucha relative partial pressure of oxygen as obtained by setting theconstant b in the expression (1) to 5.5 to 12, sintered at 1300° C. for3 hours and cooled after the sintering, and samples (recycled sinteredcores) 2-1 to 2-5 were obtained.

[0028] A quantitative analysis of Mn₂O₃ for the samples 2-1 to 2-5 thusobtained was conducted by a titration method, and the electricalresistivity and the initial permeability at 1 MHz were measured. Theresults are shown together in Table 2. TABLE 2 Sam- Electrical Initialple b Resistivity Permeability Mn₂O₃ No. Classification Constant (Ωm) 1MHz (mol %) 2-1 Comparison 5.5 9 850 0.01 2-2 Present Invention 6 801200 0.26 2-3 Present Invention 8 110 1240 0.47 2-4 Present Invention 10250 1210 0.98 2-5 Comparison 12 290 880 1.22

[0029] As apparent from the results shown in Table 2, all the samples2-2 to 2-4 of the present invention which were sintered in atmospheresof respective relative partial pressures of oxygen obtained by settingthe constant b in the expression (1) to 6, 8 and 10 have high initialpermeabilities. However, since the comparison sample 2-1 which wassintered in an atmosphere of a relative partial pressure of oxygenobtained by setting the constant b to 5.5 has a low electricalresistivity, it has a low initial permeability at a high frequencyregion of 1 MHz. And, since the comparison sample 2-5 which wasprocessed by setting the constant b to 12 contains as much as 1.22 mol %Mn₂O₃, it has a low initial permeability.

Example 3

[0030] The comparison sample 1-1 in Example 1 was milled with a hammermill and a jet mill so as to have an average grain size of 40 μm or lessto obtain a recycled powder. Then, respective raw material powders ofMnO and ZnO were added to the recycled powder so as to obtain acomposition of 49.0 mol % Fe₂O₃, and the remainder including MnO, Mn₂O₃and ZnO with a molar ratio of MnO to ZnO being 3:2 (both MnO and Mn₂O₃are counted as MnO), to thereby obtain a mixed powder. Then, this mixedpowder was mixed with a ball mill and calcined at 900° C. for 2 hours.Further, the calcined powder was milled with a ball mill to obtain afine milled powder having an average grain size of 1.27 μm. Then, 0.05mass % of MoO₃, V₂O₅, ZrO₂, CuO or Al₂O₃ was added to this fine milledpowder as additive, and further mixed with a ball mill for 1 hour. Then,this mixture was granulated with addition of polyvinyl alcohol andpressed at a pressure of 80 MPa into toroidal cores (green compacts)each having an outer diameter of 18 mm, an inner diameter of 10 mm and aheight of 4 mm. The green compacts were placed in a sintering furnacewhere an atmosphere was adjusted by flowing nitrogen so as to have sucha relative partial pressure of oxygen as obtained by setting theconstant b in the expression (1) to 8, sintered at 1300° C. for 3 hoursand cooled after the sintering, and samples 3-1 to 3-5 of the presentinvention were obtained.

[0031] A quantitative analysis of Mn₂O₃ in the samples 3-1 to 3-5 thusobtained was conducted by a titration method, and the electricalresistivity and the initial permeability at 1 MHz were measured. Theresults are shown together in Table 3. TABLE 3 Additive InitialElectrical Sample Content Permeability Mn₂O₃ Resistivity No.Classification Component (mass %) 1 MHz (mol %) (Ωm) 3-1 PresentInvention MoO₃ 0.050 1270 0.46 90 3-2 Present Invention V₂O₅ 0.050 12600.47 90 3-3 Present Invention ZrO₂ 0.050 1210 0.45 120 3-4 PresentInvention CuO 0.050 1220 0.48 120 3-5 Present Invention Al₂O₃ 0.050 12000.47 180

[0032] As can be seen from the results shown in Table 3, all the samples3-1 to 3-5 of the present invention maintain high initial permeabilityeven if a slight amount of MoO₃, V₂O₅, ZrO₂, CuO or Al₂O₃ as additive iscontained.

[0033] As described above, according to the production process of theMn—Zn ferrite of the present invention, a ferrite with sufficiently highdensity and soft magnetic properties can be obtained from wastes ofsintered cores, and the production process does not require a recycledpowder to be milled so finely Therefore, the production process of thepresent invention establishes a recycling technique that is excellent inproductivity and cost.

What is claimed is:
 1. A production process of a Mn—Zn ferrite,comprising the steps of: milling a sintered core of Mn—Zn ferrite forrecycling; subjecting a recycled powder to a component adjustment so asto have a composition of 44.0 to 49.8 mol % Fe₂O₃, 4.0 to 26.5 mol %ZnO, 0.02 to 1.00 mol % Mn₂O₃ and a remainder being MnO; pressing amixed powder subjected to the component adjustment and sintering a greencompact obtained by pressing the mixed powder.
 2. A production processof a Mn—Zn ferrite according to claim 1, wherein sintering is conductedin an atmosphere in which a relative partial pressure of oxygen isadjusted thereby controlling Mn₂O₃ content.